Adds additional default image sub-sizes.

These sizes are meant to enhance the way WordPress displays images on the front-end on larger,
high-density devices. They make it possible to generate more suitable `srcset` and `sizes` attributes
when the users upload large images.

The sizes can be changed or removed by themes and plugins but that is not recommended.
The size “names” reflect the image dimensions, so changing the sizes would be quite misleading.

function _wp_add_additional_image_sizes() {
	// 2x medium_large size.
	add_image_size( '1536x1536', 1536, 1536 );
	// 2x large size.
	add_image_size( '2048x2048', 2048, 2048 );
}